When an orthopedic surgeon performs spinal surgery involving vertebral fixation, the surgeon typically uses a pedicle awl or drill in a predrilling process to form a pilot hole in a patient's vertebrae. The surgeon then inserts pedicle screws during a screw placement process for placement of stabilization rods or plates so as to fuse two or more of the vertebrae. During this process an orthopedic surgeon typically depends on a thorough understanding of the spine's anatomy as well as an acquired knowledge of tactile feedback in order to successfully prepare the vertebra for the insertion of pedicle screws. However, due to the nature of the surgery, complications often arise that result in injury to the patient.
For example, several reports have been published describing complications associated with insertions of pedicle screws. Typically, screw misplacement includes inferior, lateral, superior, and medial breaches. Even the smallest directional error in the pedicle screw placement procedure can result in a significant error in the position of the tip of the screw. Bone disorders, such as scoliosis or osteoporosis, further complicate the procedure and lead to screw misplacement. Further, as the spine ascends, each vertebra becomes anatomically smaller. The variability in pedicle dimensions makes pedicle screw insertion a delicate operation with the potential for catastrophe. These catastrophes include breaching the cortex with either the pedicle awl in the predrilling process or with the pedicle screw during the screw placement process. An additional source of failure is when a pedicle screw breaks within the bone.
Common complications arising from misplaced pedicle screws include vascular and visceral injuries. Vascular and visceral injuries can include trauma to the aorta, vertebral arteries, azygos vein, esophagus, and lung. In addition, neurological complications may arise including injury to the spinal cord and nerve roots. These injuries can be as severe as complete paralysis to mild paresis. Neurological complications in the lumbar and lower thoracic spine typically cause paresis below the waist while complications in the cervical spine may cause paresis in the arms as well as the legs.
As another example, there are many critical structures within the thoracic cavity which make pedicle placement in the thoracic spine difficult. Critical tissues that a misplaced pedicle may come into contact with include the lung, azygos vein, the esophagus, and the aorta.
One obstacle facing physicians placing pedicles is the lack of visualization. The amount of tissue obstructing the view of the spine makes the placement of pedicles screws difficult simply because it is difficult to see the pedicle. Furthermore, the complex three-dimensional anatomy of the pedicle further complicates the process. Inter-patient variability, especially in diseased spines such as scoliosis, has resulted in imprecise placement of pedicle screws when surgeons rely on anatomic landmarks alone. In response to the need for better visualization, many companies have developed visualizations suites that help surgeons predict their trajectory when placing pedicle screws.
There are several medical device companies that have developed image guidance visualization systems for assisting in the placement of pedicle screws. In general, the systems acquire a medical image preoperatively and co-register the image with a motion capture system that reacts to the movement during the surgical process. The result is an intraoperative visualization system that depicts the pedicle screw in registration with anatomy that would not otherwise be seen during surgery.
There is no doubt that image guided visualization has helped spinal surgeons more effectively place pedicle screws. However, any breach of the cortex due to a misplaced pedicles screw is unacceptable and even with image guidance, errors are still prevalent in pedicle screw procedures. Furthermore, the systems themselves are not error free. Errors within the image guidance systems themselves could include surface generation errors, errors in fixing of the tracking devices to the patient, intraoperative data noise, registration errors, and inaccuracy of surgical probes. Therefore, surgeons understand that image guidance systems are not a substitute for skill and expertise.